Atherosclerosis reportedly affects one in four Americans, causing approximately 42% of all deaths. Recent advances in basic science have established a fundamental role for inflammation in mediating all stages of this disease from initiation through progression and ultimately, the thrombotic complications of atherosclerosis. Although expression of both vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) is upregulated in atherosclerotic lesions, VCAM-1 expression is maintained at sites of inflammation to play a dominant role in the initiation of atherosclerosis. Long-lived macrophages accumulate at sites of chronic inflammation and contribute to the inflammatory response. Our recent research provides evidence to suggest macrophages that accumulate at those sites are likely to have greater expression of inwardly rectifying K+ (Kir) currents than circulating blood monocytes (5-7). It is not known whether the presence of Kir current is a signaling event that activates monocytes and/or triggers cell differentiation. Our preliminary data suggest that a GIRKI/GIRK4 channel underlies the macrophage Kir conductance to promote an activated state that may lead to monocyte differentiation. The purpose of this project is to define the molecular basis of this Kir channel and its role in the pathogenesis of atherosclerosis. The central hypothesis of this proposal is that the macrophage Kir conductance is mediated by a tetrameric G-protein activated inward rectifier K+ channel (GIRKI/4) that plays a role in the development of macrophage foam cell formation and contributes to atherosclerotic leson formation. This central hypothesis will be tested by the following Specific Aims: 1) Identify and characterize the K+ channel species underlying VLA-4-induced inward rectifier K+ (Kir) currents in murine peripheral blood (PB)-derived monocytes and bone marrow (BM)-derived macrophages using electrophysiological, immunocytochemical and pharmacological tools and siRNAs and cDNAs for GIRK channels; 2) Determine the role of VLA-4-induced Kir activity in monocyte activation and/or differentiation to macrophages by evaluating functional and phenotypic markers of activation and differentiation; and 3) Determine the role of VLA-4-induced macrophage K+ channel activity in the pathogenesis of atherosclerosis using modified LDL uptake measurements and bone marrow transplantation in the apoE-deficient mouse model of atherosclerosis.